Suggest yarn usage for adding flow-bin

[rgbkrk]

npm -> yarn, per other guidance on initialization of a create-react-app.

[fson]

If we were to recommend yarn instead of npm in the docs, I think we should convert all npm commands at once and consistently use yarn everywhere. Having it in only some of the examples is going to be confusing, because you might not know what Yarn is and now you also need to figure that out when installing Flow, for example.

Also issues in #1189 need to be fixed before we want to recommend Yarn as the default option.

[rgbkrk]

Cool, thanks for responding to my PR so quickly. Amusingly, the reason that I filed this PR was because create-react-app has only been showing me yarn commands.

---

Steps for how I got here as a user:

Run create-react-app

$ create-react-app trial
Creating a new React app in /Users/kylek/trial.

Installing packages. This might take a couple minutes.
Installing react-scripts...

yarn add v0.18.1
info No lockfile found.
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
[3/4] 🔗  Linking dependencies...
[4/4] 📃  Building fresh packages...
success Saved lockfile.
success Saved 14 new dependencies.
├─ core-js@1.2.7
├─ encoding@0.1.12
├─ fbjs@0.8.8
├─ iconv-lite@0.4.15
├─ is-stream@1.1.0
├─ isomorphic-fetch@2.2.1
├─ loose-envify@1.3.0
├─ node-fetch@1.6.3
├─ promise@7.1.1
├─ react-dom@15.4.1
├─ react@15.4.1
├─ setimmediate@1.0.5
├─ ua-parser-js@0.7.12
└─ whatwg-fetch@1.0.0
✨  Done in 51.30s.

Success! Created trial at /Users/kylek/trial
Inside that directory, you can run several commands:

  yarn start
    Starts the development server.

  yarn run build
    Bundles the app into static files for production.

  yarn test
    Starts the test runner.

  yarn run eject
    Removes this tool and copies build dependencies, configuration files
    and scripts into the app directory. If you do this, you can’t go back!

We suggest that you begin by typing:

  cd trial
  yarn start

Happy hacking!

• Desire flow
• Flip to the flow section of the README
• Notice that instructions are npm-style rather than the yarn-style that was put on my console
• Look up the yarn syntax for adding a dev dependency
• File this PR

[fson]

The Yarn integration at the moment is opt in -- CRA only uses it if you have it installed. I can see how this transition phase is less than ideal for Yarn users because all the docs still use npm. But I hope it won't be long till we can make Yarn the recommendation for all users, at which point I think we should just convert all the examples to use it.